1. Field of the Invention
This invention relates to a process of thermomechanical treatment applicable to superalloys.
2. Description of the Background
Current economic conditions and the performance required of aircraft turbo-jet engines currently being developed have caused a return to interest in alloys with a nickel base, of the type NC 19 Fe Nb (marketed under the Registered Trademark INCONEL 718).
The relatively low cost, the absence of cobalt in the composition and experience accummulated with these alloys over many years, both by fabrication and by forging and utilisation in engines, have conferred on such alloys a preponderant position among alloys with high performance characteristics, at temperatures which may exceed 650.degree. C. for short durations.
Laboratory studies carried out by the assignee with a view to further improvement in these alloys has shown that an appreciable improvement of certain of the use characteristics, particularly the elastic limit, resistance under low cycle fatigue and creep, can be obtained by the production of fine, homogeneous, microstructures having a residual work-hardening associated with the absence of the phase Ni.sub.3 Nb-.delta. (delta) in the form of platelets. Under conventional forging conditions for these alloys, the structure of the forged parts will often appear very heterogeneous. Zones which are slighly wrought with large grains lie alongside areas of the structure which are termed "duplex" (large work-hardened grains and fine grains of recrystallisation) while the thicker parts of the test-pieces, submitted to a sufficient working and to slower cooling after forging, have a recrystallised structure with equiaxial fine grains. This observation has made clear the importance of certain thermo-mechanical treatment parameters, such as temperature, the heating period, the degree of deformation, the method of cooling, etc. in the production of the desired structure, and as a result the achievement of improved mechanical characteristics.
Thus, different ranges of thermo-mechanical treatment have been studied with a view to defining the parameters of the shaping sequences and finishing sequences which enable the development in the alloy NC19 FeNb of homogeneous structures with fine, work-hardened, grains characterized by the absence of platelets of Ni.sub.3 Nb-.delta., these results being necessarily achieved by a process applicable to the scale of industrial production.
It is important to underline that the current forging conditions for the alloy NC19 Fe Nb, effected by various forging organisations, lead to structures which represent a compromise with respect to the mechanical characteristics of the alloy. The improvement of certain properties may, in practice, give rise to the modification of other characteristics.
Thus, during the finishing sequence a re-heat temperature which is too low gives rise to the continuance of the fine structure due to the preceding sequence, with however, precipitation of the phase Ni.sub.3 Nb-.delta. in the grain boundaries or, under certain conditions, within the interior of the grains, in the form of platelets preferentially increasing in crystallographic planes of the type {111}. The phase, of orthorhombic structure, is harmful whatever its morphology since it fixes the niobium and thus limits the formation of the hardening phase Ni.sub.3 Nb-.delta." (second gamma), which is metastable, of quadratic centered structure.
Finally, in the case of the platelet morphology, the phase induces a sensitivity which is more prone to causing fatigue.
Conversely, heating, before forging, to an excessively high temperature, avoids the precipitation of .delta. phase in platelets, but leads on the contrary to an increase in the grain size, which is liable to reduce the fatigue resistance.
The main value of the process in accordance with the invention arises from the possibility of obtaining fine grained structures, in accordance with sequences of heating/forging simultaneously guaranteeing the absence of platelets of .delta. phase and the existence of residual work hardening indispensable to the consolidation of the alloy.
The beneficial influence of a fine grain structure on the fatigue resistance of INCONEL 718 is well known to the man skilled in the art. Thus U.S. Pat. No. 3,600,177 proposes a method of refining of the grain based on the precipitation of the Ni.sub.3 Nb-.delta. phase in the core of the grains before the forging operation and the recrystallisation treatment. The precipitation treatment of the phase effected at about 900.degree. C., prior to forging, leads to a subdivision of the grains by the platelets of .delta. phase which form in planes of the type {111}. The thermal treatment effected after forging with the reduction in the thickness of 50 to 65%, leads to a spheroidal phase of deformed platelets of .delta. phase and a recrystallisation of the structure. This method enables the production of recrystallised structures, of 10 ASTM or more termed "Minigrain", of which the fatigue characteristics are improved, but of which the resistence to creep and the strength are notably insufficient for a material having good characteristics, necessary for certain industrial applications.
The conditions researched in the particular case of an alloy of the type NC 19 Fe Nb marketed under the Registered Trademark INCONEL 718 apply also for superalloys with hardening by precipitation, in general, and of which those with a nickel base constitute a sub-class.
As a consequence, the present invention defines the thermodynamic parameters which enable the achievement of an improvement in all of the mechanical characteristics of these superalloys. In order to ensure industrial reproduceability and the achievement of optimum results, a rigorous control is essential during all of the fabrication processes, having regard to the forging parameters and of the thermal treatment cycles. In particular, the temperature and the amount of deformation in the finishing sequence must be well defined in order to avoid the growth of grains and the precipitation of a parasite phase, but to generate in the core of the grains a sub-structure of dislocations. In practice, in order to achieve these objectives, the method claimed by the invention must enable the satisfaction of four criteria of which known processes up to the present time do not permit simultaneous achievement:
(1) Fine and homogeneous structure;
(2) Work-hardened grains;
(3) Reduction in the stresses caused by cooling; and
(4) The absence of a parasite phase.